To meet the recent need to improve fuel efficiency, a method which improves fuel efficiency by attaching various devices to an internal combustion engine has been used. For example, an actuator for suspending cylinders which suspends some of cylinders depending on a VVT (Variable Valve Timing) for controlling the timings of intake/exhaust valves or a running condition has been known.
This may result in a complicated layout over cylinder heads so that a plug hole is extended and the foregoing devices are laid out in the resulting space. In this case, the distance between an ignition plug and the insulating case of an ignition coil increases and consequently the length of a plug boot is increased.
This also places limitations on the space around the ignition plug, whereby the ignition plug tends to have a smaller diameter.
In a conventional ignition device for an internal combustion engine, the inner diameter shape of a plug boot is determined in accordance with the diameter of an ignition plug. In addition, in accordance with the distance between the ignition plug and an ignition coil and the inner diameter shape of the plug boot, the spring constant and resonance frequency of a spring as an interposing member in the axial direction of the ignition plug are set.
There have been proposed an ignition device in which a metal pipe is inserted into a plug boot to prevent lateral vibration of a spring during the vibration of an internal combustion engine (see PTL 1) and an ignition coil in which the inner diameter of a plug boot is locally reduced to prevent the lateral vibration (PTL 2).